1.  Any cactus to any cactus, regardless of Genus.

2.  Any species to its own species.
(example: The Contorted mulberry (Morus alba cv. contorta) grafted to
a frutless mulberry (Morus Alba))

3.  Usually any Genus to another member of the same genus.  (example:
hybrid tea rose (rosa) to a wild rambling rose (rosa) or Apricot (prunus) with Almond (prunus)

4.  A bit trickier are related genera.
(example Oranges (citrus) onto Kumquat (Citrofortunella)